The present invention relates to a recirculating ball screw-nut screw coupling.
On known couplings of the aforementioned type, the nut screw is usually defined by a cylindrical body having an internal helical groove for a number of balls located between the screw and nut screw and arranged side by side along a given portion of said groove, and along a recirculating channel extending between two given points of the same.
On a first known type of screw-nut screw coupling, the recirculating channel extends outside the nut screw, each end being defined by a respective hole substantially transverse in relation to the nut screw axis and formed through the nut screw at a given point on the helical groove and substantially tangent to the same, thus enabling the balls to roll along the helical groove, into the inlet hole on the recirculating channel, and back through the outlet hole on the same to the start of the helical groove. This type of coupling undoubtedly presents excellent mechanical characteristics, by virtue of the inlet and outlet holes on the recirculating channel being perfectly aligned with the respective ends of the helical channel, thus blending the groove perfectly with the recirculating channel and substantially eliminating any possiblity of the balls gripping. Moreover, as any number of turns on the helical groove may be provided between the opposite ends of the recirculating channel, known couplings of this type may readily be designed to withstand any axial load compatible with the mechanical strength of the screw and nut screw.
Unfortunately, the relatively high manufacturing cost of couplings of this sort rules out any possibility of their being employed on widely used mass produced products, due to the transverse holes tangent to the helical groove inside the nut screw requiring extremely high precision machining, specially designed facilities, and a relatively thick nut screw.
To overcome the economic shortcomings of the above type of coupling, a second type has been devised wherein the recirculating channel is located inside the nut screw, and extends between two adjacent turns of the helical groove. On this type of coupling, the nut screw presents at least one radial hole (usually two) having a diameter equal to roughly twice the width of the internal helical groove, and located so as to involve two adjacent turns of the same. Said radial hole is normally engaged by a plug, usually made of plastic, and the inner end of which presents a groove portion blending with said two adjacent turns. In actual use, the balls roll about the screw, in contact with one another, along the turn extending between the opposite ends of the groove portion formed on the plug, then along the blend portion itself, and back to the start of the respective turn.
The economic advantages of this type of coupling as compared with the first obviously derive from the simplicity with which the radial hole is formed through the nut screw, and the fact that the cylindrical body of the nut screw itself may be extremely thin and formed, for example, from rolled sheet metal.
Though decidely cheaper to produce than the first, this type of coupling provides for relatively poor resistance to axial loads, by virtue of usually presenting only two holes and two rolling turns (one for each hole).